On the OS X client (not verified on other platforms), I am experiencing problems when I try to resize the activity feed.
The first issue I can reproduce fairly easily. The area where you can grab onto the bar to resize the column is very small, probably just one pixel, which is annoying, but not necessarily a bug. The problem is that if you pass over the area and leave that 1 pixel area, you can't the resize cursor again without having your mouse exit the window. I have been working around it by moving my mouse outside of the window and back in, but it makes it difficult to hone in on that hard-to-grab area. I found that I was able to reliably reproduce this issue.
I have a harder time coming up with exact rules for reproducing the problem, but in many cases, while trying to resize the activity feed, the cursor would actually just grab on the to the entire window and move the entire app around. I could see that the column was jittering in size as the app was simultaneously paying attention to my cursor movement as reszing the column and moving the whole app window. I was able to reliably get it to stop by putting the app into fullscreen mode (since you can't drag it around at all then) and resize the column without a problem. I can most reliably reproduce the problem by attempting to resize the feed on the first try after switching (cmd + tab) into the app. If I miss the cursor and have to try again, it doesn't seem to happen on that second try. I can usually reproduce it by switching away and back again with cmd + tab.